[SystemZ] Fix truncstore + bswap codegen bug

SystemZTargetLowering::combineSTORE contains code to transform a
combination of STORE + BSWAP into a STRV type instruction.

This transformation is correct for regular stores, but not for
truncating stores.  The routine neglected to check for that case.

Fixes a miscompilation of llvm-objcopy with clang, which caused
test suite failures in the SystemZ multistage build bot.

llvm-svn: 313669